1. Field of the Invention
This invention relates to polyisocyanates, and more particularly to blocked polyisocyanates having improved dispersibility or solubility in water.
2. Description of the Prior Art
The preparation of water-dispersible or water-soluble blocked polyisocyanates is known (see e.g. German Offenlegungsschriften Nos. 2,456,469 (corresponding to U.S. Pat. No. 4,098,933) and 2,708,611). The compounds mentioned in these publications may be used in aqueous solution or dispersion, inter alia in combination with water-dispersible or water-soluble polyhydroxyl compounds as binders for polyurethane lacquers which are applied from the aqueous phase. If, however, the polyhydroxyl compounds used are not normally soluble in water, the two components must first be modified in separate reaction mixtures to render them hydrophilic and then emulsified in water with the aid of organic cosolvents, and the two reaction mixtures are then mixed together. In the processes described in the aforesaid prior publications, moreover, a solvent-free preparation of water-dispersible or water-soluble blocked polyisocyanate is possible only with the addition of large quantities of ionic starting components or large proportions of polyethers containing ethylene oxide units as hydrophilic reaction components. This has a disadvantageous effect on the resistance to water and stability to light of the surface structures finally obtained.
It was, therefore, an object of the present invention to provide a simple, i.e. in particular a solvent-free, process for the preparation of water-dispersible or water-soluble blocked polyisocyanates which could also be used in combination with hydrophobic polyhydroxyl compounds as binders in aqueous coating compounds.
It was surprisingly found that this object could be achieved by using diaminosulphonates as hydrophilic components for the blocked polyisocyanates.